Device and method for printing with curable ink

ABSTRACT

A method for printing with curable ink is presented. The method comprises the steps of: generating an image wise pattern of spaced apart ink droplet locations for representing the image; and separating the pattern into at least first and second different interleaved portions, each portion comprising a plurality of droplet locations and the droplet locations of each interleaved portion being spaced apart from each other and spaced apart from the droplet locations of the remaining interleaved portions. In a first printing pass, ink droplets are deposited on the substrate at the drop locations of the first interleaved portion. Then, in a final printing pass, ink droplets are deposited on the substrate at the droplet locations of the second interleaved portion. The deposited ink droplets are then cured by exposing the deposited ink droplets to curing radiation in a first partial curing step between first and final printing passes and in a final curing step.

FIELD OF THE INVENTION

The method relates to the field of printing with curable ink and moreparticularly to inkjet printing with UV curable inks.

BACKGROUND

Inkjet printing is a method of printing that is well known in the art.The basics principles of this technology are described, for example, byJerome L. Johnson in “Principles of Nonimpact Printing”, Palatino Press,1992, Pages 302-336 (ISBN 0-9618005-2-6). Commercial products which useinkjet printing methodologies are widely available, including computerprinters and large format graphics printers.

An ink-jet print head typically consists of an array or matrix of inknozzles, wherein each nozzle is adapted to selectively eject inkdroplets. A given nozzle of the print head ejects an ink droplet in apredefined print position on the media. Thus, by adapting the nozzle(s)to move relative to the media, and by ejecting ink droplets in aparticular arrangement or pattern on the media, a desired pattern orimage may be printed.

Recently, curing of ink by radiation and in particular ultraviolet (UV)radiation has become popular in the field of printing. In such cases,the printer is equipped with a source of curing radiation such as a UVlamp or UV Light Emitting Diodes (LEDs) and special radiation-curableink is used. An image printed by such a printer may then be cured byexposure to the curing radiation. “Curing” in the context of the presentdisclosure means a process of converting a liquid such as a monomer andin particular ink to a solid, by exposure to radiation, which may be UVradiation.

To obtain adequate curing of the curable ink, it must be exposed to UVradiation of sufficient power. Otherwise, uneven ink curing may takeplace. For example, if the radiation does not penetrate through thewhole ink layer, a lower portion of the ink layer will not be cured,thereby leaving reactive monomers in the printed area, and interferingwith the adhesion of the ink to the media substrate.

UV radiation/light may be generated by expensive mercury-containinglamps, which consume large amounts of energy, most of which is convertedinto heat. Such lamps quickly deteriorate with the time, and thereforeneed to be replaced regularly.

Preferably, prints should be ready for use at the end of the printingprocess, meaning that the printed ink should be fully cured. As theprinting speed increases, the power of the UV lamps required to ensurecomplete ink curing to must also increase. Unfortunately, the life timeof high power lamps is shorter than lower power lamps. Furthermore, thecost of high power lamps is generally disproportionately higher thanlower power lamps. Thus, the UV lamps have become a speed-limitingfactor for inkjet printing processes that use curable ink.

Consequently, there remains a need to develop a printing method thatwill enable high-speed printing and curing with reduced UV radiationpower requirements.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, embodiments will now bedescribed, purely by way of example, with reference to the accompanyingdrawings, in which:

FIGS. 1A through 1C are schematic illustrations of prior art printingmethods;

FIGS. 2A-2C are schematic illustrations of printed ink layer thicknessas function of the ink droplet location;

FIGS. 3A-3B are schematic illustrations of an exemplary embodiment ofthe present printing method;

FIGS. 4A-4B are schematic illustrations of another exemplary embodimentof the present printing method;

FIG. 5 is a schematic illustration of an exemplary embodiment of colorprinting by the present method; and

FIG. 6 is a schematic illustration of exemplary architectures of inkjetprinters utilizing the present printing method.

Like reference characters refer to the like parts throughout thedifferent views. The drawings are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A common problem experienced when using inkjet printers in single ormulti-pass mode for color or monochromatic printing relates to thelocation of printing dots 100 or ink droplets on a substrate 104 (seeFIGS. 1A-1C). Typically, droplets are printed on top of each other or inclose proximity to one another and because of this, they can bleed intoeach other, coalesce and deform in shape. This can significantlyincrease the thickness of the ink layer on the substrate 104. Thick inklayers require significantly larger amounts of radiation curing energythan thin ink layers, since the curing radiation has to penetrate thewhole ink layer in order to cure the ink.

The printing resolution R_(p) is usually higher than the nativeresolution of the print head, or the spacing d_(ph) of the nozzles of aprint head. In order to cover the whole image area by ink, the printhead (or the media) should pass n=(R_(p)/d_(ph)) times (multi-pass forn>1) over the media (or below the print head). For example, if a desiredprint resolution R_(p) is 400 pixels per inch (or dots per inch, dpi), asingle print head with a nozzle pitch d_(ph) of 100 nozzles per inch (ordots per inch, dpi) will need to scan in a reciprocating type ofmovement (the print swath) at least four times. Each scanning movementmay be distant from the previous one by 1/400 of inch, although this candepend on the printing array resolution and the printing mode.

FIGS. 1A to 1B schematically illustrate a build-up of an image printedby a known printing method having a print resolution of 400 dpi andprint head with native resolution of 100 dpi (therefore n=400/100=4).Each circle 100 represents an ink droplet and the number inside thecircle represents the pass in which the droplet was printed. Forsimplicity of explanation, any overlap that may exist between adjacentink droplets is not shown.

To increase the media throughput rate and/or the print quality, multipleprint heads may be assembled into arrays. In such arrays, individualprint heads are located to form arrays having a resolution equal to orhigher than that of a single print head. FIG. 1C shows printing of animage by a print head array having a resolution equal to the printingresolution.

The energy required to cure a layer of deposited ink is proportional tothe thickness of the ink layer. FIGS. 2A-2C illustrate the thickness ofink layers on media 104 generated by different adjacently locatedoverlapping droplets 100. For comparison, the thickness T of an inklayer generated by a single droplet 100 (FIG. 2A) and a line 108 printedby single droplets 100 represents a basic or unity thickness. Printing aline 112 with overlapping droplets 100 (as shown in FIG. 2B) andprinting an area 116 with overlapping droplets 100 (as shown in FIG. 2C)results in ink layers having a thickness A, B of at least two 2T andmore than three 3T units, respectively. In the case each of printing anarea 116 with overlapping droplets 100 (as shown in FIG. 2C), eightneighbor droplets 100 contribute some of the ink volume to a singledroplet, thereby resulting in an ink layer of increased thickness.

The printing method according to the present invention, however, locatesprinting dots 100 on substrate 104 in a sparse arrangement. The term“sparse arrangement” means that a certain minimal distance L betweeneach of the droplets exists and is maintained in each of the printingpasses. In other words, the ink droplets deposited in a printing passare spaced apart from each other.

By maintaining a distance between the ink droplets deposited in aprinting pass, the probability of drop coalescence occurring is reducedand an increase in the ink layer thickness may be avoided. For example,when printing is performed with an equal print resolution R_(p) in bothdirections, the distance between the ink droplets deposited in aprinting pass should be at least twice that of the printing resolution,i.e. 2R_(p).

In this way, a method of inkjet printing by UV curable ink according tothe invention comprises printing droplets of UV curable inkjet ink suchthat said droplets deposit an ink volume that may be cured by a minimalUV radiation power. An ink volume that may be cured by a minimal UVradiation power or an existing (required) UV source can be defined interms of a number of droplets 100 contributing to such volume.

The droplets are ejected by one or more print heads and deposited on themedia in a pattern, such that a distance between the neighboring dropsis a maximal distance. Maximal distance means a distance wherein thedroplets are located apart from each other (preferably, as far apart aspossible) to at least prevent contact between neighboring ink dropletsdeposited on the substrate. Thus, the deposited ink droplets may form asingle ink layer on the substrate.

A UV lamp is then operated to cure the deposited ink droplets. The UV tolamp operated to emit UV radiation, and the power of the curing (UV)radiation power may be determined by the volume of deposited inkdroplets. Depending on the printer architecture (illustrated in FIGS. 6Aand 6B), UV lamps 640 may travel together with the print head 630 (FIG.6A) or be remain in a substantially stationary position (FIG. 68) over asubstrate 104. Arrows 50 indicate the direction in which the print headmoves during each pass and arrows 60 indicate a direction of substrate104 displacement. FIG. 6B shows, in a simplified way, a drum typeprinter.

The volume of deposited ink is low, the ink layer thickness is minimaland the ink drop surface is maximal and open to penetration by incidentcuring radiation. Under such conditions, the required UV curing power isreduced, preferably to a minimum, and deposited ink is rapidly cured.Thus, during the next printing path, when the print head 630 (FIG. 6A)moves in the direction indicated by arrow 70 (i.e. in a reciprocatingmovement), ink droplets are once again deposited so that they areprinted in a spaced apart arrangement. Any contact with cured inkdroplets does not affect the curing of fresh ink droplets deposited inthe more recent printing path.

FIGS. 3A-3B schematically illustrate a build-up of the image of FIGS.1A-1B printed according to one example, wherein the print resolution is400 dpi and the print head has native resolution of 100 dpi. Of course,it should be understood that the invention is applicable to any relationbetween printing resolution and print head resolution and to any printerarchitecture.

As previously explained above, each circle 100 represents an ink dropletand the number inside the circle represents the pass in which thedroplet was printed.

A pattern of spaced apart ink droplet locations is generated forrepresenting the image. This pattern is then separated into at leastfirst and second interleaved portions, the droplet locations of eachinterleaved portion being spaced apart from each other and spaced apartfrom the droplet locations of the remaining interleaved portions. In theexample shown in FIGS. 3A-3B, the pattern is separated into eightinterleaved portions.

Referring to FIG. 3A, a print head of the printer moves over thesubstrate during a first printing pass (as generally indicated by thearrow labeled “50”). During this first printing pass, the print headdeposits ink droplets 1 on the substrate 104 at the drop locations ofthe first interleaved portion. Thus, as shown in FIG. 3A, the inkdroplets 1 deposited in the first printing pass are spaced apart fromeach other.

During a next (second) printing pass, the print head reciprocates backover the substrate (as indicated by the arrow labeled “70”) and depositsink droplets 2 on the substrate 104 at the drop locations of the secondinterleaved portion. The ink droplets 2 deposited in the second printingpass are spaced apart from each other. Further, each ink droplet 2deposited in the second printing pass is spaced apart from each inkdroplet 1 deposited in the first printing pass.

From FIG. 3A, it will be understood that the ink droplets are depositedwith a substantially uniform width W and that each ink droplet depositedin a printing pass is spaced apart from the remaining droplets depositedin the same pass by a distance at least equal to the width W of an inkdroplet. Thus, the print head deposits ink droplets in a spaced apartarrangement so as to avoid the curing properties of later deposited inkdroplets affecting or being affected by previously deposited inkdroplets.

This process may then be repeated with further printing passes beingcompleted, which deposit ink droplets in a spaced apart arrangement andspaced apart from earlier deposited ink droplets. Such printing passesmay then be undertaken until no more ink droplets can be depositedwithout overlapping previously deposited ink droplets and/or a desiredimage is created from the arrangement of deposited ink droplets (asillustrated in FIG. 3B). In the example shown in FIGS. 3A-3B, eightprinting pass are completed in order to deposit ink droplets at thelocations defined by the eight interleaved portions, thereby resultingin a printed image that is represented by eight interleaved patterns ofink droplets.

The deposited ink droplets may be cured by being exposed to suitablecuring radiation. Of course, it will be understood that such curing maytake place immediately after deposition of the ink droplets so as to atleast partially cure (surface curing) deposited ink droplets of a firstprinting pass before further ink droplets are deposited in a secondprinting pass.

FIGS. 4A and 4B illustrate a build-up of an image printed according toone example, wherein the relation between the print head resolution andprinting resolution is 1:5. The printer is a drum printer (FIG. 6B)where media 104 is mounted on a rotating drum 660 and a print head 630moves in helical advance mode.

A pattern of spaced apart ink droplet locations for representing theimage is generated. The generated pattern is then separated into teninterleaved portions, wherein the droplet locations of each interleavedportion are spaced apart from each other and spaced apart from thedroplet locations of the remaining interleaved portions.

Referring to FIG. 4A, ink droplets 1 are deposited on the substrate 104at the drop locations of the first interleaved portion in a firstprinting pass. Then, in a second printing pass, ink droplets 2 aredeposited on the substrate 104 at the droplet locations of the secondinterleaved portion.

Referring now to FIG. 4B, third to tenth printing passes are completed,wherein, in each printing pass, ink droplets are deposited on thesubstrate 104 at the drop locations of the third to tenth interleavedportions, respectively.

After undertaking the ten printing passes, no more ink droplets can bedeposited without overlapping previously deposited ink droplets and thedesired image is created from the pattern of interleaved ink dropletportions.

Although the above explanation has been provided for single coloured ormonochromatic printing, the invention is also applicable to colorprinting. In another embodiment, a sparse printing pattern is formed byprinting a different amount of ink droplets in each printing pass. Somecolors that can cure more easily, such as Light Cyan (C) or LightMagenta (M), may have a higher printed proportion at a single pass thancolors that are more difficult to cure, such as black (K).

FIGS. 5A and 5B illustrate a build-up of a color image printed accordingto the invention, wherein a relation between the native print headresolution and printing resolution equal to 1:4.

FIG. 6 is a schematic illustration of different printer architectures,presented and described for better understanding of the invention. FIG.6A shows a plan view of a flat bed printer 600 where both the print head630 and the media 104 may move in a reciprocal type of movement withrespect to each other. Here, the print head 630 may be moved in alongitudinal direction (indicated by arrows 50 and 70), whereas themedia 104 may be moved in a lateral direction (indicated by arrow 60).Printing is performed by depositing ink droplets as the position of theprint head is moved relative to the media such that a generallyrectangular swath parallel to one of the movement directions is printed(i.e. the rectangular swath will be elongated longitudinally orlaterally).

FIG. 6B shows a plan view of a drum type printer 620 where media 104 iswrapped around drum 660 rotating in the direction indicated by arrow 60.While the drum 600 rotates, the print head 630 deposits ink droplets asit moves in the direction of arrow 50, thereby printing an inclined orhelical swath.

Printers 600 and 620 included an ink droplet location unit 680, whichprocesses data to be printed such that individual ink droplets depositedon the substrate are located in a sparse arrangement according to theinvention. The ink droplet location unit 680 may be implemented as asoftware or hardware and may be associated with the print headelectronics. Alternatively, it may be part of the Raster Image Processor(RIP).

When a drum printer 620 prints in a helical print mode, the ink layerprinted at the last pass may be exposed to a lower amount of UV curingradiation than ink layers printed by the earlier passes. Thus, in afurther embodiment, upon completion of printing of the image, the UVlamp 640 is not switched off and the drum 660 holding the media 104makes an additional rotation to ensure full curing of the printed image.Such an additional curing rotation may be undertaken during a rotationof the drum 660 that is used for unloading the media 104, therebyincreasing the exposure time to UV curing radiation (and therefore thesupplied energy) without increasing the overall UV system power.

In another embodiment, the rotation speed of the drum 660 may bedecreased during the last pass in order to increase the amount of timethat the ink printed in the last pass is exposed to curing radiation.

The same may be true for flat bed printing machines where the print heador the bed supporting the substrate move in helical or step like manner.It is desirable for the printed ink droplets to be fully cured uponcompletion of printing, and therefore at least one additional pass maybe made over the final printed ink with only UV lamp operating.

An embodiment of the invention provides a method of printing an image ona substrate using a curable ink, wherein the method comprises the stepsof: irrespective of the image to be printed, generating a pattern ofspaced apart ink droplet locations for representing the image;separating the pattern into at least first and second differentinterleaved portions, each portion comprising a plurality of dropletlocations and the droplet locations of each interleaved portion beingspaced apart from each other; in a first printing pass, depositing inkdroplets on the substrate at the drop locations of the first interleavedportion; in a final printing pass, depositing ink droplets on thesubstrate at the droplet locations of the second interleaved portion;and curing said deposited ink droplets by exposing the deposited inkdroplets to curing radiation in a first partial curing step between thefirst and final printing passes and in a final Curing step.

While specific embodiments have been described herein for purposes ofillustration, various modifications will be apparent to a person skilledin the art and may be made without departing from the scope of theinvention.

The invention claimed is:
 1. A method of printing an image on asubstrate using a curable ink, the method comprising: generating animage wise pattern of spaced apart ink droplet locations forrepresenting the image; separating the pattern into at least first andsecond different interleaved portions, each portion comprising aplurality of droplet locations and the droplet locations of eachinterleaved portion being spaced apart from each other; in a firstprinting pass, depositing ink droplets on the substrate at the droplocations of the first interleaved portion; in a final printing pass,depositing ink droplets on the substrate at the droplet locations of thesecond interleaved portion; and curing said deposited ink droplets byexposing the deposited ink droplets to curing radiation in a firstpartial curing step between the first and final printing passes and in afinal curing step, wherein the first partial curing step is performedonly after the first printing pass has been completed, wherein no curingis performed in either the first printing pass or the final printingpass, and wherein curing of the ink droplets deposited in the finalprinting pass occurs only during the final curing step.
 2. A method ofprinting according to claim 1, wherein the deposited ink droplets aredeposited with a substantially uniform width and each deposited inkdroplet is spaced apart from the remaining droplets of ink deposited inthe same printing pass by a distance at least equal to the width of adeposited ink droplet.
 3. A method according to claim 1, wherein thecurable ink is ultra-violet, UV, curable ink that is curable by exposureto UV radiation.
 4. A method according to claim 1, wherein saiddeposited ink droplets form a single ink layer on the substrate.
 5. Amethod according to claim 1, wherein the final curing step comprises,after the final printing pass, moving the substrate past a source ofcuring radiation such that said deposited ink droplets are exposed tocuring radiation one or more times to ensure full curing of thedeposited ink droplets.
 6. A non-transitory computer-readable datastorage medium having a computer program stored thereon, whereinexecution of the computer program by a computing device causes a methodto be performed, the method comprising: generating an image wise patternof spaced apart ink droplet locations for representing the image;separating the pattern into at least first and second differentinterleaved portions, each portion comprising a plurality of dropletlocations and the droplet locations of each interleaved portion beingspaced apart from each other; in a first printing pass, depositing inkdroplets on the substrate at the drop locations of the first interleavedportion; in a final printing pass, depositing ink droplets on thesubstrate at the droplet locations of the second interleaved portion;and curing said deposited ink droplets by exposing the deposited inkdroplets to curing radiation in a first partial curing step between thefirst and final printing passes and in a final curing step, wherein thefirst partial curing step is performed only after the first printingpass has been completed, wherein no curing is performed in either thefirst printing pass or the final printing pass, and wherein curing ofthe ink droplets deposited in the final printing pass occurs only duringthe final curing step.
 7. Apparatus for printing an image on a substrateusing a curable ink, the apparatus comprising: an ink droplet locationunit adapted to generate a pattern of spaced apart ink droplet locationsfor representing the image and to separate the pattern into at leastfirst and second different interleaved portions, each portion comprisinga plurality of droplet locations and the droplet locations of eachinterleaved portion being spaced apart from each other; a print headarranged to deposit ink droplets on the substrate at the drop locationsof the first interleaved portion; a print head arranged to deposit inkdroplets on the substrate at the drop locations of the secondinterleaved portion; and curing means for curing said deposited inkdroplets by exposing the deposited ink droplets to curing radiation in afirst partial curing step between first and final printing passes and ina final curing step, wherein the first partial curing step is performedonly after the first printing pass has been completed, wherein no curingis performed in either the first printing pass or the final printingpass, and wherein curing of the ink droplets deposited in the finalprinting pass occurs only during the final curing step.
 8. Apparatusaccording to claim 7, wherein the deposited ink droplets are depositedwith a substantially uniform width and each deposited ink droplet isspaced apart from the remaining droplets of ink deposited at locationsof the same interleaved portion by a distance at least equal to thewidth of a deposited ink droplet.
 9. Apparatus according to claim 7,wherein the curable ink is ultra-violet, UV, curable ink that is curableby exposure to UV radiation and the curing means are adapted to emit UVradiation.
 10. Apparatus according to claim 7, wherein the apparatus isarranged to deposit the ink droplets such that they form a singledroplet ink layer on the substrate.
 11. Apparatus according to claim 7,wherein the apparatus is adapted to move the substrate past the curingmeans in the final curing step such that said deposited ink droplets areexposed to curing radiation one or more times to ensure full curing ofthe deposited ink droplets.
 12. Apparatus according to claim 7, whereinthe apparatus is a printer.